1. Field of the Invention
This invention relates to a plasma addressed liquid crystal display device having a two-layer structure including two layers of a liquid crystal cell and a plasma cell, and more particularly to a gap controlling structure for a liquid crystal cell of a plasma addressed liquid crystal display device of the type mentioned.
2. Description of the Related Art
A liquid crystal display device of the matrix type conventionally employs, as commonly known as means for assuring a high resolution and a high contrast, an active matrix addressing system wherein a switching element such as a thin film transistor is provided for each picture element and the switching elements are driven in a line sequential condition. However, according to the active matrix addressing system, it is necessary to provide a large number of semiconductor elements such as thin film transistors on a substrate. Accordingly, the active matrix addressing system is disadvantageous in that, when the substrate has a large area, the yield in production is low.
A solution to the disadvantage has been proposed by Buzak et al. and is disclosed in Japanese Patent Laid-Open Application No. Heisei 1-217396 wherein a plasma switch is employed in place of a switching element formed from a thin film transistor or a like element. Now, an outline of general construction of a plasma addressed liquid crystal display device wherein a liquid crystal cell is driven making use of a switch based on plasma discharge will be descried. Referring to FIG. 11, the plasma addressed liquid crystal display device shown has a layered flat panel structure which includes a liquid crystal cell 101, a plasma cell 102 and a common intermediate sheet 103 interposed between the liquid crystal cell 101 and the plasma cell 102. The plasma cell 102 is formed using a glass substrate 104 and has a plurality of striped grooves 105 formed on a surface thereof. The grooves 105 extend, for example, in the direction along a row of a matrix. The grooves 105 are individually closed up by the intermediate sheet 103 to define plasma chambers 106 which are individually separate from each other. Ionizable gas is enclosed in the plasma chambers 106. A convex portion 107 of the glass substrate 104 in the form of a rib is disposed between each adjacent ones of the grooves 105 and serves as a barrier rib for isolating the adjacent plasma chambers 106 from each other. A pair of parallel discharge electrodes are provided on a curved bottom surface of each of the grooves 105 and function as an anode A and a cathode K to ionize the gas in the corresponding plasma chamber 106 to produce discharge plasma. Such discharge area makes a row scanning unit.
Meanwhile, the liquid crystal cell 101 is constructed using a transparent substrate 108. The substrate 108 is disposed in an opposing relationship to the intermediate sheet 103 with a predetermined gap left therebetween, and a liquid crystal layer 109 is filled in the gap. Signal electrodes 110 are formed on an inner surface of the substrate 108. The signal electrodes 110 extend perpendicularly to the plasma chambers 106 and make column driving units. Picture elements in a matrix are defined at intersecting positions between the column driving units and the row scanning units.
In the display device having such a construction as described above, the plasma chambers 106 in which plasma discharge occurs are selectively scanned in a line sequential condition while an image signal is applied to the signal electrodes 110 of the liquid crystal cell 101 in synchronism with such scanning to effect display driving of the display device. If plasma discharge occurs in a plasma chamber 106, then the potential of the inside of the plasma chamber 106 is put substantially uniformly to that of the anode A so that picture element selection of the row is performed. In other words, each of the plasma chamber 106 functions as a sampling switch. If an image signal is applied to a picture element of a plasma sampling switch while the plasma sampling switch is in an on state, then sampling holding takes place so that lighting or extinction of the picture element can be controlled. Also after the plasma sampling switch is put into an off state, the image signal is held as it is in the picture element.
In the conventional plasma addressed liquid crystal display device of the structure described above, the discharge electrodes are formed on the curved bottom surfaces of the grooves 105 such that an anode A and a cathode K in each pair are disposed in an opposing relationship to each other in an inclined condition. In this structure, a route of plasma discharge is formed between the electrode surface of an anode A to the electrode surface of an opposing cathode K, and accordingly, comparatively stable plasma discharge can be obtained. However, in order to realize such electrode structure as described above, it is necessary to form the striped grooves 105 on the surface of the substrate 104, but this involves considerable difficulty in production, and particularly it is very difficult to provide a stripe pattern in a high density. Also, it is complicated and difficult to actually form discharge electrodes in the individual grooves 105 making use of an etching process.
In order to solve such drawbacks of the conventional plasma addressed liquid crystal display devices, a plasma addressed liquid crystal display device which is easy to manufacture and is suitably used to produce a screen of a large size and/or a high resolution has been proposed and is disclosed, for example, in Japanese Patent Laid-Open Application No.
Heisei 4-265931. The structure of the plasma addressed liquid crystal display device will be described subsequently with reference to FIG. 12.
Also the display device has a flat panel structure wherein a liquid crystal cell 201 and a plasma cell 202 are layered with each other with an intermediate sheet 203 interposed therebetween. The liquid crystal cell 201 has a basically same structure as the liquid crystal cell 101 shown in FIG. 11. Ionizable gas is enclosed between the intermediate sheet 203 and a lower substrate 204 to form a plasma chamber 205. A plurality of striped discharge electrodes 206 are formed on an inner surface of the substrate 204. Since the discharge electrodes 206 can be formed on a flat substrate by screen printing or a like technique, the productivity and the operability are high and the discharge electrodes 206 can be formed finely. A barrier rib 207 is formed on each of the discharge electrodes 206, and the barrier ribs 207 divide the plasma chamber 205 into several discharge regions which make row scanning units. Also the barrier ribs 207 can be formed by screen printing or a like technique, and the top ends thereof contact with the lower surface of the intermediate sheet 203. The striped discharge electrodes 206 alternately function as an anode A and a cathode K and cause plasma discharge between them.
Also with the plasma addressed liquid crystal display device of the structure shown in FIG. 12, there remains a subject to be solved. For example, it is difficult to arrange the striped barrier ribs 207 in a fixed height as seen from FIG. 13. Consequently, the intermediate sheet 203 which are supported by the top ends of the barrier ribs 207 cannot be held flat and the thickness of the liquid crystal layer 208 is varied locally. Such irregularity in thickness of the liquid crystal layer 208 significantly deteriorates the quality of an image displayed by the liquid crystal layer 208. Particularly, since the intermediate sheet 203 which isolates the liquid crystal cell 201 and the plasma cell 202 from each other has a very small thickness of approximately 50 .mu.m, it is deformed readily and cannot maintain its flatness.
In order to control the gap of the liquid crystal cell 201 uniformly, it is a conventional countermeasure to spray particulates 209 of a fixed particle size at random. Such particulates 209 are present in the gap and can act effectively to some degree against deformation in a compression direction to keep the dimension of the gap fixed. However, where the density of the particulates 209 sprayed is restricted comparatively low, they cannot sometimes bear the compression force. Meanwhile, as for deformation in an expansion direction, the particulates 209 cannot keep the dimension of the gap fixed. Normally, gas which is ionizable under the pressure lower than the atmospheric pressure is enclosed in the plasma cell 202 on the lower side of the intermediate sheet 203. Accordingly, the intermediate sheet 203 is inclined to be deformed downwardly by a negative pressure to increase the dimension of the gap. In this instance, even if the particulates 209 are sprayed at random so as to be present in the gap, they float in the liquid crystal layer 208 and do not function effectively.